1. Field of the Invention
The present invention relates to the field of anti-scratch coatings for spectacle glass and more particularly anti-scratch coatings loaded with mineral particles.
2. Description of the Related Art
It is quite common, in the field of spectacle glasses, to use anti-abrasion and/or anti-scratch coatings to improve the surface properties of organic glasses.
Most coatings of this type are composed of alkoxysilane hydrolysates and in particular epoxysilane hydrolysates.
Colloidal loads, such as colloidal silica or mineral oxides, such as titanium oxide, zirconium oxide, tin oxide or antimony oxide, with high refraction index, are widely used in protective coatings for spectacle glasses. Their presence enables to reinforce the hardness of the grid formed by the epoxysilane matrix.
It is possible to incorporate such loads at high contents in the coating, i.e. up to approx. 50 to 60% in weight of dry matter of the coating.
Although the coated ophthalmic lenses available on the market are generally satisfactory, it is advisable to increase the hardness and the scratch-resistance properties of these existing coatings.
This problem is particularly delicate.
Indeed, the increase in the colloid ratio, which enables to reinforce the hardness of the coating, involves making said coating more rigid and more brittle, so that the gain obtained in hardness, does not translate, at high colloid ratios, by greater scratching resistance for the coating/substrate system, taken as a whole.
Besides, the incorporation of very high colloid quantities (more than 60% in weight) also brings about other shortcomings, such as for example cracking problems due to an expansion difference between the coat of mineral nature and the substrate, or of other coats, essentially organic in nature, applied previously to the surface of the spectacle glass.
The colloids used in the commercial coatings are colloids with globally isotropic particles, i.e. substantially spherical.
The synthesis of anisotropic colloids is known and their applications have been described in several patents or patent applications.
The patent U.S. Pat. No. 5,403,513 describes the synthesis TiO2 colloids with anisotropic particles, with a form factor L/D varying from 1 to 20.6. (L being the length in the greater main axis of the particle and D the length in the shorter axis of the particle).
This document mentions that the colloid may be utilised as a surface coating material for transparent articles, made of mineral glass or plastics.
No specific formulation for the coating is described and it is the deposition of the sol only as a solution which is recommended.
The patent U.S. Pat. No. 5,407,600 describes nanoparticles whereof the length represents approx. 5 to 10 times the thickness thereof.
The patent mentions that the sols have a natural tendency to form films.
The Japanese patent JP 64941 (publication no. JP 63-225635) describes coatings resistant in particular to light degradation, usable on spectacle glasses and sun glasses.
The coatings include a thermosettable resin obtained from an organosilane which may be γ-glycidoxypropyltrimethoxysilane (Glymo) and non-agglomerated titanium dioxide particles whereof the form factor L/D varies from 1 to 10, and the particles are cubical or parallelepipedal in shape.
In the example 1 of this patent, the particles have a ratio L/D equal to 2.
Compared with a titanium colloid coating of conventional form factor, the UV-resistance is described as being considerably increased for the coating with the colloid of the patent.
The coating also shows good resistance to abrasion.
The Japanese patent JP 59-78925 describes an alumina sol comprising a pseudo-boehmite whereof the crystal is needled-shaped with diameter smaller than or equal to 10 millimicrons and 200 millimicrons in lengths.
The sol is introduced in films which are in fact PVA-films.
No optical application is mentioned.
The Japanese patent application JP 7133463 describes a composition for the preparation of a film which is transparent, hard and shows good adherence to a substrate comprising a hydrolysate of an organoalkoxysilane, a metal colloid, among other constituents.
The patent application mentions that the particles of the colloid may be spherical or cylindrical.
The colloid may be the Aluminasol® 100, 200 or 520.
There is no mention of the possible influence of the form factor of the particles.
The patent application published EP-1 245 968 describes a laminated article comprising a two-coat antiglare film comprising an outer layer applied on an inner layer comprising particles of a needle-shaped, mixed tin/antimony oxide. The inner layer is based on (meth)acrylate urethane.
It is specified that the needle shape includes forms of fibres, bars and columns.
These particles have an aspect ratio of 5 or more, preferably of 10 or more.
These particles are used at high contents (from 40 to 80% in weight) in the outer layer. The patent application EP-1 245 968 specifies that below 40% in weight, it is not possible to obtain good electrostatic and scratch-resistance properties.
Even if some of these documents suggest or recommend, in certain cases, the use of anisotropic particles in abrasion-resistant coatings, none of these documents suggest using very high form factor particles, i.e. greater than 10, in a polysiloxane-based coating.